FIG. 1 illustrates a single turn induction coil 102 surrounding a metal strip 104 that continuously moves through the coil in the direction indicated by the arrow. Terminals 106 and 108 of coil 102 are connected to a suitable ac power source with instantaneous current flow through the coil as indicated by the arrows. The current flowing through the coil establishes a magnetic field that inductively heats the strip as it passes through the coil. However unless some type of electromagnetic shielding is used, the generated magnetic field intensities in regions beyond the region in which the coil and strip are magnetically coupled may exceed permissible levels.
One known method of reducing field intensities in the desired regions is to place electrically conductive screens 110 and 111, such as a rectangular copper form, around the strip as shown in FIG. 1. As understood from Faraday's law, instantaneous current flow induced by the magnetic field in each screen will be opposite in direction to the instantaneous current flow in the induction coil. In the arrangement in FIG. 1, current will flow in the same direction in the upstream (element 111) and downstream (element 110) screens. In FIG. 2, two single turn coils 102a and 102b are used with instantaneous current flow in opposite directions established by connecting terminals 106a and 108a, and terminals 106b and 108b, to one or more suitable ac power supplies. With the arrangement of FIG. 2, current flow will be in opposite directions in the upstream and downstream screens. One disadvantage of the arrangements in FIG. 1 and FIG. 2 is that each screen is closed around the strip, which makes lateral movement of the strip in and out of the screens impossible. Such lateral movement is desirable for changing induction coils without cutting a continuous workpiece.
U.S. Pat. No. 5,034,586 discloses a method of using decoupling rings (elements 20 and 22) with a switch (element 24) in the rings. The switch opens to allow a workpiece to be moved laterally out of, or into, the decoupling rings. The decoupling rings reduce inter-coil magnetic coupling between adjacent induction coils. One disadvantage of this method is that an electrically conductive component, namely the decoupling rings, must include a mechanical switch that reduces reliability and increases operational complexity and maintenance.
Therefore there is the need for a means of electromagnetic shielding of an induction coil, through which a moving workpiece passes to be inductively heated, that would allow the workpiece to laterally move in and out of the electromagnetic shielding without the use of mechanical switches.